Loss of cardiomyocyte CYB5R3 impairs redox equilibrium and causes sudden cardiac death

Sudden cardiac death (SCD) in patients with heart failure (HF) is allied with an imbalance in reduction-oxidation (redox) signaling in cardiomyocytes; however, the basic pathways and mechanisms governing redox homeostasis in cardiomyocytes are not fully understood. Here, we show that cytochrome b5 reductase 3 (CYB5R3), an enzyme known to regulate redox signaling in erythrocytes and vascular cells, is essential for cardiomyocyte function. Using a conditional cardiomyocyte-specific CYB5R3 knockout mouse, we discovered that deletion of CYB5R3 in male adult cardiomyocytes, but not female, causes cardiac hypertrophy, bradycardia, and SCD. The increase in SCD in CYB5R3 KO mice is associated with calcium mishandling, ventricular fibrillation, and cardiomyocyte hypertrophy. Molecular studies reveal that CYB5R3 KO hearts display decreased ATP, increased oxidative stress, suppressed coenzyme Q levels, and hemoprotein dysregulation. Lastly, from a translational perspective, we reveal that the high-frequency missense genetic variant rs1800457, which translates into a CYB5R3 T117S partial loss-of-function protein, associates with decreased event-free survival (~20%) in African Americans with HF with reduced ejection fraction (HFrEF). Together, these studies reveal a crucial role for CYB5R3 in cardiomyocyte redox biology and identify a new genetic biomarker for individuals in the African American population that may potentially increase the risk of death from HFrEF. Overall design: Conditional cardiac muscle-specific CYB5R3 knockout mice (CYB5R3fl/fl, Myh6-CreERT2) and wild-type mice (CYB5R3wt/wt, Myh6-CreERT2) received tamoxifen injections between the age of 10-13 weeks. The whole heart of 6 knockout and wild-type mice (n=6) was collected for RNA sequencing analysis.